I use "cup grease" on grease cups such as on my 1910 T u joint. I also use it on other grease cups as on the blower fan on my Lenox furnace at my store. I needed some and went to buy a can of cup grease at a local hardware. They did not even know what I was talking about. They had greases such as bearing grease and marine grease, but no cup grease. I went home and searched my storage area of my garage and found a 5 pound can of cup grease from the old Pure Oil Company. I have heard that if new grease, such a wheel bearing grease is used , one has to clean out all the old grease before adding new bearing grease. If old and new are mixed, the combination will cause the mixture to become liquid and will run out. I heard this at a Model A Ford clinic by a grease salesperson. Is true cup grease still available? What grease do you use in grease cups?
I don't think it is a problem of old and new grease but rather if the two are made of different base materials. Some types are very much not compatible. Many greases out there can be used on our Ts but one must remain loyal to one type and stick with it to avoid the compatibility problem.
This is what Wile E Coyote recommends.
"What grease do you use in grease cups?"
I use (chassis) grease gun grease from a grease tube.
I believe you can obtain cup grease from Restoration Supply in Escondido, California.
The only thing you have to worry about is if you mix petroleum based grease with something synthetic like Lithium...it turns hard and develops a crust, that over time you'd have to damn near break with a chisel or soak it for a long time in automatic transmission fluid.
Doing a little backwards research, I looked up grease cups on the McMaster Carr website. In the grease cup description, it says to fill them with NLGI #1 - #3 grease. The various greases offered by McMaster have their NLGI number shown. I would choose a general purpose grease within the 1 - 3 spec. rating.